The data lines are good up to the voltage level of the Vcc line.So if power from 3.3V, your data out lines will be ~3.3V and any incoming lines should be <=3.3V.If you power from 5V, same thing: outputsp will be ~5V, and any coming lines should be <=5V.

The data lines are good up to the voltage level of the Vcc line.So if power from 3.3V, your data out lines will be ~3.3V and any incoming lines should be <=3.3V.If you power from 5V, same thing: outputsp will be ~5V, and any coming lines should be <=5V.

Thanks for replying. Well that was my point on the one USB to Serial device I have. You cannot jumper it to put out 3.3 or 5 volts but it has a Vcc reading 3 volts and another Vcc that reads 5 volts. The Rx/Tx read 5 volts either way.

So use the 5 volt Vcc line then?

What would happen if I used the Vcc 3 volts and the data lins Rx/Tx are putting out +5?

Without a schematic, it is going to be tough to solve this strange puzzle. Since it doesn't look like there is a regulator onboard, it could be that the 3V3 isn't VCC, but a regulated output from the PL2303.

"What would happen if I used the Vcc 3 volts and the data lins Rx/Tx are putting out +5?"Tx won't put out 5V, it would only put 3V.If the other end drove Rx to 5V, then the Rx pin would get damaged - the uC only tolerates Vcc + 0.5V. 5V is well above 3.5V.

Thanks again for the replies. I had a link to the product I bought in my first post. I measured each line. Vcc +5 measures +5. Vcc 3.3 measures 3.288 Rx and Tx measure just under 5 volts each. I used the ground as reference when measuring each pin.

Something sounds wrong with this device.

I have another USB to serial that has a jumper one for +5 and other for +3.3 so they measure correctly. When +5 is enabled I measure Rx/Tx juts under 5 volts and when I enable +3.3 I measure just under 3.3 volts for the Rx/Tx using ground for reference.